Audio band width control



June 1, 1943.

H. BOUCKE AUDIO BAND WIDTH CONTROL Filed April 5, 1941 2 Sheets-Sheet lFREQUENCY (crews) INVENTOR HE/NZ ,BOl/C'KE ATTORNEY June 1, 1943.

AUDIO 5e .S/GNALS LAAA SIGNALS l H. BOUCKE AUDIO BAND WIDTH CONTROL'Filed April 3, 1941 2 Sheets-Sheet 2 INVENTOR usmz 1:21am; BY v ATTORNEYiatente d June 1, 1943 J NIT a sparse Fi th AUDIO mm) Wr rn CONTROLApplication April 3, 1941, Serial. No. 386,653 In Germany January 16,1940 6 Claims.

My present invention relates to an audio frequency band width controlwith the aid of an additional tube.

It has been proposed in the past to insert as a shunt into the audiofrequency section of an audio frequency amplifier, the cathode to anodepath of a tube with controlled amplification and with voltagecounter-coupling dependent on frequency.

As compared with this arrangement the advantage of the present inventionresides in the fact that the direct plate current of the control tube isdecoupled from the alternating current circuit in which the tube acts asa shunt. Therefore, a high ohmic design of the alternating voltagedivider is possible. Furthermore, this ofi'ers the possibility of aneasy additional provision of the control element in an already operatingamplifier circuit.

In accordance with the present invention the input resistance of a tubeprovided with a counter-coupling path connected between its anode andgrid serves as a frequency-dependent parallel resistance in the audiofrequency section, or it represents a part of such a parallelresistance.

The phenomenon known as such is hereby utilized that the inputresistance of a tube with voltage counter-coupling from the anodedirectly to the grid decreases with an increase of the degree ofcounter-coupling. For the purpose of varying the degree of thecounter-coupling the grid biasing potential of the tube having a controlcharacteristic is varied, or a control of the current distribution iscarried out.

Examples of construction according to the present invention are shown inthe accompanying figures, wherein Fig. 1 shows an embodiment of theinvention; Fig. 2 graphically shows the operation thereof; and Fig. 3 isa modification; Fig. 4 shows the operation of Fig. 3, and Fig. 5 showsanother modification.

.In Fig. 1 the alternating input potential Ee is applied to the grid ofthe tube V1. The input may be audio waves. The alternating potentialdeveloped across the plate resistance R1 is applied to the voltagedivider formed by C1, R2, R3 and C5. The potential E9. tapped at theresistance R3 serves as the output potential. The condensers C1, C2 andC6 are chosen with such a high value that they represent a negligiblylow resistance for all frequencies. The chosen values may be, for28.3:500 to 1000 kil-ohms, R4 together with C2 represent thecounter-coupling path, and R4 is instance, Rz==300 kilo-ohms,

about 1 to 3 megohm. The series circuit L-C:

may be tuned, for instance, to 1 lrilocycle. In that case an alternatingplate potential appears at Rs which is the higher the more it liesabove, or below, the short circuit frequency of the circuit LC3. Thedegree of the counter-coupling of the circuit, hence, has a minimum forthe frequencies at 1 kllocycle. Since a voltage-dependentcounter-coupllng of the input resistance of the counter-coupling circuitreduces, in the present case, the value of the resistance R3 in a mannerknown as such, R3 has its actual value only for the frequencies in therange of 1 kilocycle approximately. Below this value R3 appearsdecreased to an increasing degree.

There corresponds with this frequency-dependent behavior of R3 theoutput potential Ea.-

The frequency curve extends approximately horizontal for a high negativegrid biasing potential Er. Hence, at a small amplification of the tubeV2, (when disregarding the direct influence oi the counter-couplingpath), the curve deviates the more towards both sides from 1 kilocycle(see Fig. 2), the higher the control action of the degree of theamplification of V2. There corresponds with the curve A in Fig. 2 zeroamplification of the tube V2, and with the curve I) maximumamplification- The curves B and C represent intermediate values.

' The circuit of Fig. 3 differs from that of Fig. 1 in that in theformer the circuit elements L-C3 are connected as a parallel resonantcircuit which is tuned for instance to 9 kilocycles. This denotes thatthe counter-coupling occurs primarily in the region of this highfrequency, i. e. R3 appears greatly decreased for these frequencies,whereas for the intermediate frequencies and for the low frequencies itretains its actual value. By varying the grid biasing potential Er,therefore, the effectiveness of the 9 kilocycle blocking means (andtherewith, according to Fig. 4, the band width) will be influenced. Thecurves A to G of Fig. 4 correspond with an increasing amplification ofthe tube V2 of Fig. 3.

In Fig. 5 the counter-coupling in the tube V2 has approximately the samevalue for all frequencies. The variation of the band width occures inview of the fact that the frequencydependent resistance short circuitsthe high frequencies the more effectively the lower the apparent valueof R3 with increasing amplification of the tube. When dimensioning C2 insuch a manner that the counter-coupling path proper is effectivepredominantly for the high frequencies, a multiplicative action of thecontrol influence can be attained and, therewith, a particularly steepdescent of the flank of the curve. The same is true for an arrangementaccording to'Fig. 3. Obviously it, is also possible so to design thecounter-coupling paths in a manner known as such, that besides theprimary change of the frequency curve in the range of the highfrequencies. there takes place at the same time acontrol of the lowfrequencies, especially in the sense that a slight drop of the lowfrequencies occurs at a constricting of the band width. This isaccomplished in Fig. 3, for instance, by inserting the RC network anddesignated by R7-C5. This network is so chosen that for the frequenciesbelow 200 cycles a counter-coupling potential develops which increaseswhen frequency decreases.

What is claimed:

1. In an audio amplifier having input and output terminals, an impedanceelement across the output terminals, an impedance simultation tubehaving input electrodes connected across said element whereby the inputtube resistance between said input electrodes is effectively in shuntwith said impedance element, a'resonant circuit tuned to a predeterminedaudio frequency in circuit with the tube output electrodes to cause theinput resistance to have a desired freqency response characteristic, andmeans for varying the tube gain thereby to varysolely the magnitude ofthe input resistance. I

2. In an audio amplifier as defined in claim 1. said resonant circuitbeing tuned to a frequency of substantially 1,000 cycles whereby theaudio voltage of audio frequencies other than 1,000 cycles developedacross said impedance element decreases in magnitude in response toincrease of said tube gain.

3. In an audio amplifier as defined in claim 1, said tuned audiofrequency being substantially 9,000 cycles whereby said responsecharacteristic is a low pass filter response for high tube gain.

4. In an audio amplifier having input and output terminals, an impedanceelement across the output terminals, an impedance simulation tube havinginput electrodes connected across said element, a resonant circuit tunedto a predetermined audio frequency in circuit with the tube outputelectrodes to cause the simulated impedance to have a desired frequencyresponse characteristic, means providing degenerative audio feedbackfrom the output electrodes to said input electrodes, means for varyingthe tube gain thereby to vary solely the magnitude of the simulatedimpedance, and a condenser-resistor network in circuit with saidresonant circuit for reducing the development ofvoltage, of low audiofrequencies.

5. In an audio transmission system, a source of audio voltage, an outputload element, a resistive element in shunt therewith, a tube forcontrolling the effective magnitude of said resistive element, said tubehaving input and output electrodes, said tube input'electrodes beingconnected across said resistive element, a degenerative audio voltagefeedback path between'the output and input electrodes, means forcontrolling the gain of said tube whereby the input resistance betweenthe input electrodes, in shunt with said resistive element, is variable,and a tuned circuit in series between said tube output electrodes, saidtuned circuit being resonant to a predetermined audio frequency.

6. In an audio transmission system, a source of audio voltage, an outputload element, a resistive element in shunt therewith, a tube forcontrolling the efiective magnitude of said resistive element, said tubehaving input and output electrodes, said tube input electrodes beingconnected across said resistive element, a degenerative audio voltagefeedback path betwen the output and input electrodes, means forcontrolling the gain of said tube whereby the inputresistancebetween'the input electrodes, in shuntwith said resistive element, isvariable, and a tuned circuit in series between said tube outputelectrodes, said tuned circuit being resonant to a predetermined audiofrequency, and an aperiodic network in series with the tuned circuit fordiscriminating against the low audo frequencies.

HEINZ BOUCKE.

